1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Recently, there is known a color electrophotographic technology for controlling glossiness of an image by superimposing a clear toner image on four-color toner images of yellow, magenta, cyan, and black.
For example, in Japanese Patent Application Laid-open No. 2009-058941, clear-toner usage is controlled according to a print mode. For example, if a print image is a monochrome image, the clear toner is not used in a monochrome print mode, or printing is performed with suppressing clear-toner usage more than that in a color print mode. Likewise, there is disclosed a technology for controlling clear-toner usage according to a mode such as a photo image mode, a toner saving mode, and a high-speed printing mode. When the print image is a monochrome image, the clear toner is not used in the monochrome print mode or printing is performed with suppressing clear-toner usage more than that in the color print mode, and this allows an image with optimal gloss to be output.
In addition, Japanese Patent Application Laid-open No. 2008-129547 discloses an image forming apparatus that includes color-image forming units for forming color images using a plurality of chromatic toners, and a clear-toner image forming unit for forming a clear toner image. The image forming apparatus has an image forming mode in which a text portion is formed with the clear toner, and forming conditions to form a latent image of the clear toner image in the text portion in image data can be changed. As this apparatus, there is invented an image forming apparatus capable of outputting an image having glossiness that user desires by changing an adhesion amount of the clear toner per unit area according to image input information for controlling the level of glossiness.
Furthermore, in Japanese Patent Application Laid-open No. 2004-070010, a glossiness detector for detecting glossiness of an image on a sheet is provided in a post-process of a fixing device. As test patterns for measuring glossiness, three-color gray gradation patterns (patches) in which a toner adhesion amount is largely changed are created, and the glossiness of each patch is detected by the glossiness detector. Parameters on image formation such as fixing conditions are set so as to reduce a difference in the glossiness between the patches (images). Although the clear toner is not used in this apparatus, there is invented a technology for equalizing glossiness from halftone to a solid portion by detecting the glossiness of an output image and changing the fixing conditions.
As explained above, there exist the technologies for controlling the glossiness by changing the adhesion amount of the clear toner or the like, however, none of the inventions are technologies for controlling gloss or glossy feeling perceived by humans.
First, the glossiness will be explained below before the glossy feeling is discussed. The glossiness is a physically measured quantity indicating intensity of a light amount obtained such that light is irradiated to a surface and is reflected thereon in a specular direction. A measurement method of glossiness is defined in JIS-Z8741. The measurement method of glossiness is performed by causing a parallel light to enter a sample object to be evaluated at a defined angle of incidence θ, detecting a light flux reflected on the sample object in the specular direction by a light receiver, and normalizing the detected reflected light flux using a reflected light flux detected on a standard plane (which is a glass surface whose refractive index over an entire range of visible wavelength is 1.567) under the same condition as above.
The measurement method of glossiness defines measurement methods in which 20°, 45°, 60°, 75°, and 85° are adapted as an angle of incidence θ. In general, a method for measuring light incident at a small angle of incidence is preferably used to measure an object to be evaluated with high specular gloss. On the contrary, a method for measuring light incident at a large angle of incidence is preferably used to measure an object to be evaluated with low specular gloss. In a case of an electrophotographic image, 20° and 60° are often used as the angle of incidence θ. In order to quantify subjective glossy feeling perceived by humans, the measurement of glossiness is widely implemented.
However, many literatures point out that the glossiness and the subjective glossy feeling do not necessarily coincide with each other. The reason is because when humans feel gloss, they do not perceive gloss only by the intensity of specular light amount from a sample but perceive gloss including the degree of spread of the reflected light.
The spread of the reflected light will be explained below with reference to FIGS. 1A and 1B. Among lights reflected from a sample, the intensity of the light in the specular direction is the highest. The lights reflected on the sample include some lights diffused on a surface other than that in the specular direction. If the reflected light is widely diffused on the surface, as shown in FIG. 1A, the amount of the reflected light near the specular direction is also high, and thus the spread of the reflected light is wide. Conversely, because diffused reflection on the surface is narrow in FIG. 1B, the spread of specular reflected light is narrow.
Humans perceive glossy feeling based on not only the intensity of the specular reflected light but also the spread of the reflected light in the above manner, and, therefore, it is difficult to quantify the glossy feeling only by the glossiness. Therefore, there is widely known a method of measuring image clarity in order to quantify the spread of reflected light. The image clarity is a gloss characteristic, which is observed on the surface of a high-gloss image such as silver halide photography, indicating the intensity of a light source reflected into the surface of the screen. The image clarity affects not only the silver halide photography but also the glossy feeling of an image formed by an electrophotographic image forming apparatus or an ink-jet image forming apparatus. As the method of measuring image clarity, “JIS K 7105” and “JIS H 8686” or the like are used.
When the spread of the reflected light is wide, the image clarity is low, but when the spread of the reflected light is narrow, then the image clarity is high. As explained above, if not only the level of the glossiness but also the level of the image clarity is not controlled, it is difficult to output an image with glossy feeling the user desires.
Next, advantages of controlling the glossiness and the image clarity that determine the glossy feeling will be discussed below. The glossy feeling can be largely divided into the following four classifications such as A, B, C, and D.
A. Case of Low Image Clarity and Low Glossiness
In this case, the image clarity is low and the glossiness is low. Users frequently handling text files such as office documents prefer this case. If the glossiness and the image clarity of text are high and when the light from a fluorescent light is reflected on the text, then the reflected light enters user's eyes, which causes the user to feel glaring and the user's eyes get tired very much. To avoid the glare, this type of glossy feeling is required for a graph or the like which is inserted into the office document.
B. Case of Low Image Clarity and High Glossiness
In the case where the image clarity is low and the glossiness is high, reflection of an image is low because of the low image clarity. Thus, when the user looks at an image, a reflection into the image, which is a reflection of the face or the like of the user, is low. On the other hand, because the glossiness is high, this allows glaring to be given. Therefore, when a particular image is desired to be emphasized while a face is not desired to be reflected into the particular image, the glossy feeling categorized as the case B is required. For example, when advertisement in a train is desired to be accentuated but faces of passengers are desired to be avoided from being reflected thereinto, the classification B is effective. In addition, in most cases, the glossiness is high but the image clarity is low in output images on high-quality paper used in offset printing. When an image like that in offset printing is desired to be output, an image having the glossy feeling categorized as B is required.
C. Case of High Image Clarity and Low Glossiness
In this case, the image clarity is high and the glossiness is low. This is effective when an image, which resembles an image on a gloss paper or the like formed by an inkjet printer, is desired to be output. In electrophotography, an image having both high glossiness and high image clarity can be obtained by cold release, while in conventional electrophotography, it is difficult to output an image having high image clarity but low glossiness. On the contrary, an inkjet printer can output an image having low glossiness and high image clarity depending on various types of gloss papers. Such an image is soft and expensive looking, but this image is difficult to be output in electrophotography. If the user desires the glossy feeling like that of the image printed by the inkjet printer using the gloss paper, the glossy feeling categorized as C has to be given.
D. Case of High Image Clarity and High Glossiness
In this case, the image clarity is high and the glossiness is high. This case is effective when the glossy feeling like that of a silver halide photography is desired to be given. If the glossy feeling with high glossiness and expensive looking like in silver halide photography or a print by a digital camera is desired to be given, the glossy feeling of D is preferred. The conventional electrophotography allows the glossy feeling when the cold release technology is used.
As explained above, all the four categories have to be satisfied in order to control the glossy feeling of the image. However, in many cases, the conventional electrophotographic image forming apparatuses can control only A and D using the cold release technology, and thus, there does not exist any electrophotographic image forming apparatus capable of expressing all the glossy feelings. Particularly, there is no electrophotographic image forming apparatus that can independently control image clarity from glossiness.
The present invention has been achieved to solve the conventional problems, and an object of this invention is to provide an image forming apparatus that can output an image having glossy feeling a user desires by controlling image clarity using clear toner.